A mixed-phase structured layered oxide and its preparation method and application

Abstract

The present invention provides a layered oxide with a mixed-phase structure, its preparation method, and its application, belonging to the technical field of sodium-ion battery positive electrode materials. This invention coats an O3 phase material with a layer of P2 phase material. This coating structure combines a high-capacity core O3 phase with a structurally stable outer P2 phase, improving the material's reversibility and air stability, reducing surface residual alkali, and improving rate performance. Furthermore, this invention eliminates the need for lithium doping to control the formation of the composite phase, reducing production costs.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery positive electrode materials, and in particular to a layered oxide with a mixed phase structure, a preparation method and an application thereof. Background Art

[0002] The abundance and widespread distribution of sodium resources give sodium-ion batteries the advantage of low cost. Therefore, sodium-ion batteries have attracted widespread attention and are considered to be an effective supplement and alternative to lithium-ion batteries, especially in the field of energy storage. Among the positive electrode materials of sodium-ion batteries, layered transition metal oxides have become the main research focus due to their simple structure, easy synthesis, environmental friendliness and excellent electrochemical performance. + Coordination configuration and oxygen polyhedron stacking sequence, layered oxide positive electrode materials are divided into different phases such as P2, O2, P3 and O3. Generally speaking, the electrochemical performance of P2 and O3 phases is better. The O3 phase has a higher sodium content and usually has a higher theoretical capacity, but because Na ions need to overcome a higher energy barrier to pass through the tetrahedral gap, the electrochemical kinetics of most O3 phase materials are poor, and the phase change process is complex, resulting in rapid capacity decay. In addition, most O3 phase materials have poor air stability. When exposed to humid air, residual alkali (NaOH and Na2CO3) will be formed to destroy the material performance. P2 phase materials have better rate performance and better structural stability because the diffusion channels in the prismatic space are wider and sodium ions are easy to diffuse; but due to the lower sodium content, their gram capacity is lower. In order to combine the advantages of the two phases, some researchers have synthesized P2 / O3 intergeneration Na by Li substitution. 0.8 Li 0.2 Ni 0.5 Mn 0.5 Patent CN113651368A discloses a method for preparing P2 phase materials and P2 / O3 mixed phase materials by regulating the amount of Li doping. However, lithium resources are scarce, expensive, and have low economic value. Summary of the Invention

[0003] The purpose of the present invention is to provide a layered oxide with a mixed phase structure, a preparation method and application thereof, which combines a high-capacity core O3 phase and a structurally stable shell P2 phase, improves the reversibility and air stability of the material, reduces surface residual alkali and improves rate performance. In addition, the present invention does not require Li doping to regulate the formation of a composite phase, thereby reducing production costs.

[0004] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0005] The present invention provides a layered oxide with a mixed phase structure, comprising an O3 phase material and a P2 phase material coated on the surface of the O3 phase material;

[0006] The O3 phase material has a chemical composition shown in Formula 1:

[0007] Na x Ni a Fe b Mn c M d O 2±β Formula 1,

[0008] In Formula 1, Ni is divalent, Fe is trivalent, and Mn is trivalent and / or tetravalent; M is an ion that is doped and substituted for one or more of the three transition metals Ni, Fe, and Mn; M includes Mg 2+ 、Cu 2+ 、Zn 2+ 、Al 3+ 、B 3+ 、Co 3+ 、Y 3+ 、Ti 4 + 、Zr 4+ 、Sn 4+ and Nb 5+ One or more of;

[0009] In formula 1, x, a, b, c, d and 2±β are the molar numbers of the corresponding elements, 0.67<x≤1; a+b+c+d=1, 0≤d<1;

[0010] The P2 phase material has a chemical composition shown in Formula 2:

[0011] Na y Mn e G f O2 formula 2,

[0012] In formula 2, y, e and f are the molar numbers of the corresponding elements, Mn is trivalent and / or tetravalent; G includes Mg 2 + 、Cu 2+ 、Zn 2+ 、Ni 2+ and Co 2+ One or more of the following, 0<y≤0.67, e+f=1, 0<e≤1.

[0013] Preferably, the mass ratio of the P2 phase material to the O3 phase material is 0.02 to 0.15:1.

[0014] The present invention provides a method for preparing the layered oxide described in the above scheme, comprising the following steps: dispersing an O3 phase material in ethanol to obtain a solution A;

[0015] Dissolve polyvinyl pyrrolidone in ethanol to obtain solution B;

[0016] Adding the B solution to the A solution to form a PVP coating layer on the surface of the O3 phase material to obtain a complex solution;

[0017] According to the theoretical composition of Na, Mn and G in formula 2, sodium acetate, manganese acetate and acetate containing element G are dissolved in ethanol to obtain solution C;

[0018] Under heating conditions, the C solution is added to the complex solution, and after the ethanol is completely volatilized, the obtained solid is calcined to generate a P2 phase material on the surface of the O3 phase material to obtain a layered oxide with a mixed phase structure.

[0019] Preferably, the preparation method of the O3 phase material comprises the following steps: according to the theoretical composition of Ni, Fe and Mn in Formula 1, mixing a water-soluble salt of divalent Ni, a water-soluble salt of divalent Fe and a water-soluble salt of divalent Mn with water to obtain a mixed metal salt solution;

[0020] The mixed metal salt solution, alkali metal hydroxide solution and ammonia water are mixed, and a coprecipitation reaction is carried out at a pH value of 10 to 12, followed by aging to obtain a hydroxide precursor;

[0021] mixing the hydroxide precursor and a sodium source to obtain a solid mixture;

[0022] Alternatively, the hydroxide precursor, the sodium source, and the M source are mixed to obtain a solid mixture;

[0023] The solid mixture is sequentially subjected to a first sintering, a heating, and a second sintering to obtain an O3 phase material; the temperature of the first sintering is 400-500° C.; the temperature of the second sintering is 800-950° C.;

[0024] The mass of Na in the sodium source is 2-5% in excess of the theoretical mass of Na calculated according to Formula 1; the mass of M in the M source is the theoretical mass of M calculated according to Formula 1.

[0025] Preferably, the total concentration of sodium acetate, manganese acetate and acetate containing element G in the C solution is (0.001-1) g / mL.

[0026] Preferably, the heating temperature is 60-80°C.

[0027] Preferably, the calcination temperature is 800-850° C. and the holding time is 10-15 hours.

[0028] Preferably, the calcination is carried out in air or oxygen atmosphere.

[0029] Preferably, the mass ratio of polyvinyl pyrrolidone in the B solution to the O3 phase material in the A solution is 0.005-0.05:1.

[0030] The present invention provides the use of the layered oxide described in the above scheme or the layered oxide prepared by the preparation method described in the above scheme as a positive electrode material in a sodium ion battery.

[0031] The present invention coats an O3 phase material with a layer of P2 phase material. This coating structure combines a high-capacity core O3 phase with a structurally stable outer P2 phase, enhancing the material's reversibility and air stability, reducing surface residual alkali, and improving rate performance. Furthermore, the present invention eliminates the need for lithium doping to control the formation of the composite phase, reducing production costs.

[0032] The present invention provides a method for preparing a layered oxide having a mixed-phase structure as described in the above-mentioned scheme. The additive polyvinylpyrrolidone (PVP) selected by the present invention has a number of excellent properties, including low toxicity, film-forming properties, complexing properties, surface activity, and chemical stability. PVP has a long chain structure, and the carbonyl oxygen on the molecular chain can donate a pair of electrons to metal cations, or form complex chemical bonds between the nitrogen in the five-membered nitrogen-containing heterocycle and the metal ion. In addition, due to the wettability of PVP, PVP can pair with the metal ions on the surface of the original O3 phase material in an ethanol solvent to form a uniform PVP coating layer. Subsequently, metal acetate is added, and the dissolved metal ions are also adsorbed by the PVP coating layer to form a thin metal acetate layer. As a polymer compound, PVP has a high thermal decomposition temperature, and its structure is not destroyed during the solvent evaporation process, and the complexed metal acetate layer can be well preserved. Finally, through high-temperature calcination, it reacts with some residual alkali on the surface of the O3 phase material to form a P2 phase, which can greatly reduce the amount of residual alkali on the surface of the original O3 phase material.

[0033] The present invention generates a P2 phase material on the surface of an O3 phase material through liquid phase coating and high-temperature sintering, thereby realizing a P2 phase material with controllable coating thickness and adjustable composition on the surface of the O3 phase material (the thickness of the coating layer can be controlled by controlling the amount of PVP added). The method has the advantages of a simple process route and easy operation, and does not use expensive raw materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a scanning electron microscope test image of the material obtained in Example 1 of the present invention;

[0035] Figure 2 This is a scanning electron microscope test image of the material obtained in Example 2 of the present invention;

[0036] Figure 3 This is a scanning electron microscope test image of the material obtained in Example 3 of the present invention;

[0037] Figure 4 This is a scanning electron microscope test image of the material obtained in Comparative Example 1 of the present invention;

[0038] Figure 5 The X-ray powder diffraction test patterns of the materials obtained in Example 1, Example 2, Example 3 and Comparative Example 1 of the present invention are shown;

[0039] Figure 6 This is a charge and discharge curve diagram of the sodium ion battery provided in Example 1 of the present invention;

[0040] Figure 7 This is a charge and discharge curve diagram of the sodium ion battery provided in Example 2 of the present invention;

[0041] Figure 8 This is a charge and discharge curve diagram of the sodium ion battery provided in Example 3 of the present invention;

[0042] Figure 9 This is a charge and discharge curve diagram of the sodium ion battery provided in Comparative Example 1 of the present invention;

[0043] Figure 10 These are test diagrams of the cycling performance of sodium ion batteries at different rates provided in Examples 1, 2, 3, and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0044] The present invention provides a layered oxide with a mixed phase structure, comprising an O3 phase material and a P2 phase material coated on the surface of the O3 phase material;

[0045] The O3 phase material has a chemical composition shown in Formula 1:

[0046] Na x Ni a Fe b Mn c M d O 2±β Formula 1,

[0047] In Formula 1, Ni is divalent, Fe is trivalent, and Mn is trivalent and / or tetravalent; M is an ion that is doped and substituted for one or more of the three transition metals Ni, Fe, and Mn; M includes Mg 2+ 、Cu 2+ 、Zn 2+ 、Al 3+ 、B 3+ 、Co 3+ 、Y 3+ 、Ti 4 + 、Zr 4+ 、Sn 4+ and Nb 5+ One or more of;

[0048] In formula 1, x, a, b, c, d and 2±β are the molar numbers of the corresponding elements, 0.67<x≤1; a+b+c+d=1, 0≤d<1;

[0049] The P2 phase material has a chemical composition shown in Formula 2:

[0050] Na y Mn e G f O2 formula 2,

[0051] In formula 2, y, e and f are the molar numbers of the corresponding elements, Mn is trivalent and / or tetravalent; G includes Mg 2 + 、Cu 2+ 、Zn 2+ 、Ni 2+ and Co 2+ One or more of the following, 0<y≤0.67, e+f=1, 0<e≤1.

[0052] The O3 phase material will be described below.

[0053] In the present invention, the O3 phase material has a chemical composition as shown in Formula 1: Na x Ni a Fe b Mn c M d O 2±β Formula 1. In Formula 1, x can be 0.75, 0.8, 0.9, or 1; d can be 0, 0.05, or 0.1; and β can be 0, 0.05, or 0.1. In the present invention, the O3 phase material satisfies the valence conservation law and is electrically neutral.

[0054] In an embodiment of the present invention, the chemical composition of the O3 phase material is NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0055] The present invention does not impose any special restrictions on the specific position of the substitution of M.

[0056] In the present invention, the particle size of the O3 phase material is preferably 4 to 10 μm.

[0057] The P2 phase material is described below.

[0058] In the present invention, the P2 phase material has a chemical composition shown in Formula 2: Na y Mn e G f O2 formula 2.

[0059] In Formula 2, y can be specifically 0.67; e can be specifically 0.7 or 1; and f can be specifically 0.3. In an embodiment of the present invention, the P2 phase material is specifically Na 2 / 3 MnO2 or Na 2 / 3 Mn 0.7 Mg 0.3 O2

[0060] In the present invention, the mass ratio of the P2 phase material to the O3 phase material is preferably 0.02 to 0.15:1, more preferably 0.05 to 0.1:1.

[0061] The present invention coats an O3 phase material with a layer of P2 phase material. This coating structure combines a high-capacity core O3 phase with a structurally stable outer P2 phase, enhancing the material's reversibility and air stability, reducing surface residual alkali, and improving rate performance. Furthermore, the present invention eliminates the need for lithium doping to control the formation of the composite phase, reducing production costs.

[0062] The present invention provides a method for preparing the layered oxide described in the above scheme, comprising the following steps:

[0063] Dispersing the O3 phase material in ethanol to obtain solution A;

[0064] Dissolve polyvinyl pyrrolidone in ethanol to obtain solution B;

[0065] Adding the B solution to the A solution to form a PVP coating layer on the surface of the O3 phase material to obtain a complex solution;

[0066] According to the theoretical composition of Na, Mn and G in formula 2, sodium acetate, manganese acetate and acetate containing element G are dissolved in ethanol to obtain solution C;

[0067] Under heating conditions, the C solution is added to the complex solution, and after the ethanol is completely volatilized, the obtained solid is calcined to generate a P2 phase material on the surface of the O3 phase material to obtain a layered oxide with a mixed phase structure.

[0068] In the present invention, unless otherwise specified, all raw materials used are commercially available products well known in the art.

[0069] The present invention has no special requirements on the source of the O3 phase material, and it can be prepared using methods well known in the art.

[0070] In the present invention, the preparation method of the O3 phase material preferably includes the following steps:

[0071] According to the theoretical composition of Ni, Fe and Mn in Formula 1, a water-soluble salt of divalent Ni, a water-soluble salt of divalent Fe and a water-soluble salt of divalent Mn are mixed with water to obtain a mixed metal salt solution;

[0072] The mixed metal salt solution, alkali metal hydroxide solution and ammonia water are mixed, and a coprecipitation reaction is carried out at a pH value of 10 to 12, followed by aging to obtain a hydroxide precursor;

[0073] mixing the hydroxide precursor and a sodium source to obtain a solid mixture;

[0074] Alternatively, the hydroxide precursor, the sodium source, and the M source are mixed to obtain a solid mixture;

[0075] The solid mixture is sequentially subjected to a first sintering, a heating, and a second sintering to obtain an O3 phase material; the temperature of the first sintering is 400-500° C.; the temperature of the second sintering is 800-950° C.;

[0076] The mass of Na in the sodium source is 2-5% in excess of the theoretical mass of Na calculated according to Formula 1; the mass of M in the M source is the theoretical mass of M calculated according to Formula 1.

[0077] According to the theoretical composition of Ni, Fe and Mn in Formula 1, the present invention mixes a water-soluble salt of divalent Ni, a water-soluble salt of divalent Fe and a water-soluble salt of divalent Mn with water to obtain a mixed metal salt solution.

[0078] In the present invention, the water-soluble salt of divalent Ni, the water-soluble salt of divalent Fe and the water-soluble salt of divalent Mn are independently preferably one or more of chloride, nitrate, sulfate, acetate and citrate; in an embodiment of the present invention, the water-soluble salt of divalent Ni is nickel sulfate, the water-soluble salt of divalent Fe is ferrous sulfate, and the water-soluble salt of divalent Mn is manganese sulfate.

[0079] The present invention has no special requirements for the mixing process, as long as the water-soluble salt of divalent Ni, the water-soluble salt of divalent Fe and the water-soluble salt of divalent Mn can be completely dissolved.

[0080] In the present invention, the total concentration of metal ions in the mixed metal salt solution is preferably 1 to 3 mol / L, more preferably 1.5 to 2.5 mol / L.

[0081] After obtaining the mixed metal salt solution, the present invention mixes the mixed metal salt solution with an alkali metal hydroxide solution and aqueous ammonia, performs a coprecipitation reaction at a pH value of 10 to 12, and ages the mixture to obtain a hydroxide precursor.

[0082] In the present invention, the alkali metal hydroxide solution is preferably obtained by dissolving an alkali metal hydroxide in water; the alkali metal hydroxide is preferably sodium hydroxide or potassium hydroxide; the concentration of the alkali metal hydroxide solution is preferably 2 to 5 mol / L, more preferably 3 to 4 mol / L. In the present invention, the alkali metal hydroxide solution serves as a precipitant.

[0083] In the present invention, the concentration of the ammonia water is preferably 4 to 6 mol / L, more preferably 5 mol / L. In the present invention, the ammonia water serves as a complexing agent.

[0084] In the present invention, the mixed metal salt solution, the alkali metal hydroxide solution and the ammonia water are preferably added into the reaction kettle simultaneously and in parallel, and the pH value of the system is maintained at 10 to 12 to carry out the coprecipitation reaction.

[0085] The present invention has no special requirements on the dosage of the alkali metal hydroxide solution and ammonia water, as long as the pH value of the reaction system is stabilized to 10-12.

[0086] In the present invention, the temperature of the precipitation reaction is preferably 50-60° C., more preferably 52-56° C. In the precipitation reaction process of the present invention, the metal ions first form a complex with ammonia water, and then react with the alkali metal oxide to form a spherical metal hydroxide.

[0087] The present invention has no special requirements on the precipitation reaction time, as long as the average particle size of the hydroxide precursor is 4 to 10 μm.

[0088] In the present invention, the aging time is preferably 10 to 30 hours, more preferably 15 to 25 hours. In the present invention, dissolution and crystallization achieve a dynamic equilibrium during the aging process, the reaction tends to proceed in the direction of low energy, the residual metal ions in the system continue to react on the surface of the metal hydroxide, causing the small particles to grow further, and promoting rearrangement within the crystals, thereby improving the crystallinity of the metal hydroxide precipitate.

[0089] After the aging is completed, the present invention preferably filters the aged product system, washes it with deionized water four times, and then dries it at 110° C. for 10 h to obtain a precursor powder.

[0090] After obtaining the spherical hydroxide precursor, the present invention mixes the hydroxide precursor with a sodium source to obtain a solid mixture, or mixes the hydroxide precursor, a sodium source and an M source to obtain a solid mixture.

[0091] In the present invention, the mass of Na in the sodium source is 2-5% excess over the theoretical mass of Na calculated according to Formula 1; the present invention controls the excess mass of Na to compensate for Na burnout. In the present invention, the mass of M in the M source is the theoretical mass of M calculated according to Formula 1.

[0092] In the present invention, the sodium source is preferably one or more of sodium nitrate, sodium peroxide, sodium superoxide, sodium carbonate, sodium hydroxide and sodium oxalate, more preferably sodium carbonate or sodium hydroxide.

[0093] In the present invention, the M source is preferably one or a combination of oxides, borides, hydroxides, oxyhydroxides, carbonate compounds, nitric acid compounds and acetic acid compounds containing the M element, more preferably oxides, hydroxides or oxyhydroxides.

[0094] The present invention has no special requirements on the mixing method for obtaining the solid mixture, as long as the substances can be mixed uniformly.

[0095] After obtaining the solid mixture, the present invention sequentially performs a first sintering, a temperature increase, and a second sintering on the solid mixture to obtain the O3 phase material.

[0096] In the present invention, the temperature of the first sintering is 400-500°C, preferably 420-480°C; the holding time is preferably 3-6 hours, more preferably 4-5 hours; the heating rate from room temperature to the first sintering temperature is preferably 2-6°C·min -1 , more preferably 3 to 5°C·min -1 In the first sintering process of the present invention, the metal hydroxide precursor undergoes a thermal decomposition reaction to generate water and metal oxides, and removes impurities from the solid mixture, which is beneficial for the subsequent slow penetration of the sodium source into the crystal after decomposition, making the reaction more uniform, thereby facilitating the subsequent formation of a layered structure and improving the crystallinity.

[0097] In the present invention, the rate of heating from the first sintering temperature to the second sintering temperature is preferably 2-6°C·min -1 , preferably 2~5℃·min -1 .

[0098] In the present invention, the temperature of the second sintering is 800-950°C, preferably 850-900°C; the holding time is preferably 10-20 hours, more preferably 12-18 hours, and even more preferably 14-16 hours. In the present invention, the first and second sintering are preferably performed in an air atmosphere.

[0099] In the second sintering process of the present invention, the sodium source melts and participates in the crystallization reaction, and M doping replaces part of the transition metal sites.

[0100] After completing the second sintering, the present invention preferably cools to room temperature, and grinds the obtained material through a 250-mesh screen to obtain the O3 phase material.

[0101] After obtaining the O3 phase material, the present invention disperses the O3 phase material in ethanol to obtain solution A.

[0102] In the present invention, the ethanol is preferably anhydrous ethanol. In the present invention, the solid-liquid ratio of the O3 phase material to ethanol is preferably 1 g:1.5-2.5 mL. In the present invention, the dispersion is preferably carried out under ultrasonic conditions, and the ultrasonic time is preferably 10 minutes.

[0103] In the present invention, polyvinyl pyrrolidone is dissolved in ethanol to obtain solution B. In the present invention, the concentration of PVP in the solution B is preferably 8 to 16 g / L.

[0104] After obtaining solution A and solution B, the present invention adds the solution B to the solution A under stirring conditions to form a PVP coating layer on the surface of the O3 phase material to obtain a complex solution.

[0105] In the present invention, the mass ratio of polyvinyl pyrrolidone in the B solution to the O3 phase material in the A solution is preferably 0.005-0.05:1, more preferably 0.01-0.04:1, and further preferably 0.02-0.03:1.

[0106] In the present invention, the B solution is preferably added dropwise. The present invention has no special requirements for the rate of addition, and can be added dropwise. In the present invention, PVP has a long chain structure, and the carbonyl oxygen on the molecular chain can provide a pair of electrons to the metal cation, or form a complex chemical bond between the nitrogen in the five-membered nitrogen-containing heterocycle and the metal ion, and due to the wettability of PVP, PVP can be paired with the metal ions on the surface of the original O3 phase material in the ethanol solvent to form a uniform PVP coating. By adjusting the amount of PVP, those skilled in the art can adjust the thickness of the coating.

[0107] According to the theoretical composition of Na, Mn and G in formula 2, sodium acetate, manganese acetate and acetate containing the G element are dissolved in ethanol to obtain a C solution.

[0108] In the present invention, the sodium acetate is preferably sodium acetate trihydrate; the manganese acetate is preferably manganese acetate tetrahydrate; and the ethanol is preferably anhydrous ethanol. In the present invention, the total concentration of sodium acetate, manganese acetate, and acetate containing element G in solution C is preferably (0.001-1) g / mL, more preferably (0.01-0.9) g / mL, further preferably (0.1-0.8) g / mL, and most preferably (0.2-0.7) g / mL.

[0109] After obtaining the C solution, the present invention adds the C solution to the complex solution under heating conditions, waits for the ethanol to completely volatilize, and calcines the obtained solid to generate a P2 phase material on the surface of the O3 phase material to obtain a layered oxide with a mixed phase structure.

[0110] In the present invention, the heating temperature is preferably 60-80°C. In the present invention, the C solution is preferably added dropwise. After the metal acetate is added, the dissolved metal ions are also adsorbed by the PVP coating layer to form a thin metal acetate layer. As a polymer compound, PVP has a high thermal decomposition temperature, and its structure is not destroyed during solvent evaporation, effectively preserving the complexed metal acetate layer.

[0111] Before the calcination, the solid is preferably crushed in the present invention.

[0112] In the present invention, the calcination temperature is preferably 800-850°C, more preferably 810-830°C; the holding time is preferably 10-15 hours, more preferably 12-13 hours. In the present invention, the calcination is preferably carried out in air or an oxygen atmosphere. During the calcination process, the thin metal acetate layer reacts with some residual alkali on the surface of the O3 phase material to form the P2 phase, which can significantly reduce the amount of residual alkali on the surface of the original O3 phase material.

[0113] The present invention provides the use of the layered oxide described in the above scheme or the layered oxide prepared by the preparation method described in the above scheme as a positive electrode material in a sodium ion battery.

[0114] The mixed-phase layered oxide provided by the present invention, its preparation method and application are described in detail below with reference to the examples, but they should not be construed as limiting the scope of protection of the present invention.

[0115] In the following examples and comparative examples, the O3 phase material is NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, the preparation method is as follows:

[0116] Ferrous sulfate, manganese sulfate and nickel sulfate were weighed in a molar ratio of nickel, iron and manganese of 1 / 3:1 / 3:1 / 3, and dissolved in deionized water to prepare a metal salt solution with a concentration of 1 mol / L. The precipitant sodium hydroxide was prepared into a sodium hydroxide solution with a concentration of 5 mol / L, and the complexing agent ammonia water was prepared into a solution with a concentration of 5 mol / L.

[0117] A mixed metal salt solution, sodium hydroxide solution and ammonia water are added to the reactor simultaneously and in parallel, the pH is maintained at 10-12, and a coprecipitation reaction is carried out. After the particle size reaches 4-10 μm, it is aged and allowed to stand for 12 hours, filtered, washed with deionized water 4 times, and dried at 110° C. for 10 hours to obtain a precursor powder.

[0118] The above Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 The (OH)2 precursor was uniformly mixed with the required stoichiometric amount of 105 wt% sodium carbonate and transferred to a corundum crucible. The mixture was calcined in a muffle furnace under air atmosphere. The first stage was heated at a rate of 5 °C·min under air atmosphere. -1 , keep warm at 500℃ for 5 hours, and the heating rate in the second stage is 3℃·min -1 , keep warm at 880℃ for 12 hours, and then cool to room temperature; grind the calcined material through a 250-mesh sieve to obtain O3 phase material.

[0119] Example 1

[0120] Coated with 0.5wt% P2-Na 2 / 3 Preparation of MnO2 layered oxide positive electrode material (NM-0.5, i.e., P2 phase material is 0.5wt% of O3 phase material, the rest of the embodiments have the same meaning and will not be repeated):

[0121] Take 20g NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 Add 50 mL of ethanol to the O2 solution, ultrasonically disperse for 5 minutes, and continue stirring to form solution A. Take 0.2 g of PVP and completely dissolve it in 20 mL of ethanol to form solution B, which is then added dropwise to the stirring solution A to obtain a complex solution.

[0122] Dissolve 0.2396g of Mn(CH3COO)2·4H2O and 0.0888g of C2H3O2Na·3H2O in 20mL of ethanol as Solution C. Use a pipette to drip Solution C dropwise into the complex solution. Stir at 80°C until the ethanol is completely evaporated. The resulting precipitate is pulverized and calcined at 810°C for 11 hours in air.

[0123] Example 2

[0124] Coated with 1wt% P2-Na 2 / 3 Preparation of MnO2 layered oxide cathode material (NM-1):

[0125] Take 0.4792 g of Mn(CH3COO)2·4H2O and 0.1766 g of CH3COONa·3H2O and dissolve them in 30 mL of ethanol as solution C. The remaining steps are the same as those in Example 1.

[0126] Example 3

[0127] Coated with 1wt% P2-Na 2 / 3 Mn 0.7 Mg 0.3 Preparation of O2 layered oxide positive electrode material (NMM-1): 0.3687gMn(CH3COO)2·4H2O, 0.1382gC4H 14 MgO8·4H2O and 0.1949 g CH3COONa·3H2O were dissolved in 30 mL of ethanol as solution C. The remaining steps were the same as those in Example 1.

[0128] Comparative Example 1

[0129] O3 phase layered cathode material NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0130] Structure and performance characterization:

[0131] The morphology of the prepared layered positive electrode materials was analyzed using a field emission scanning electron microscope (SEM) (Hitachi Regulus 8100). The scanning electron microscope images of the layered oxide positive electrode materials prepared in Examples 1 to 3 and Comparative Example 1 are shown in FIG. Figure 1 、 Figure 2 、 Figure 3 and Figure 4 As shown. In Comparative Example 1, NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 The surface of the O2 primary particles is relatively smooth and has a clear outline. The coated 0.5 wt% P2-Na 2 / 3 The MnO2 layered oxide positive electrode material (NM-0.5) has a clear primary particle morphology due to the small amount of coating, but the coating is obvious at the edge of the primary particle. The coating of 1wt% P2-Na prepared in Example 2 2 / 3MnO2 layered oxide positive electrode material (NM-1) and the coated 1wt% P2-Na prepared in Example 3 2 / 3 Mn 0.7 Mg 0.3 The layered oxide cathode material (NMM-1) of O2 has a large amount of coating, so the primary particle morphology is blurred and the primary particles are obviously filled with coatings.

[0132] The materials of Examples 1 to 3 and Comparative Example 1 were analyzed by XRD diffractometer (Holland Panalytical X'PERT PRO MPD). The XRD patterns of the layered oxide positive electrode materials prepared in Examples 1 to 3 and Comparative Example 1 are as follows: Figure 5 As shown, all are typical O3-type phase structures (JCPDS Card No. 54-0887). Due to the low coating amount, the XRD patterns of Examples 1, 2, and 3 do not show diffraction peaks of other phases. However, the intensity and sharpness of the diffraction peaks are slightly reduced compared to Comparative Example 1. The sharp diffraction peaks indicate that they are highly crystalline.

[0133] Furthermore, the residual alkali content on the surface of the material was tested using a potentiometric titration method. The test results are shown in Table 1.

[0134] Table 1 Residual alkali content on the surface of layered oxide materials prepared in Examples 1 to 3 and Comparative Example 1

[0135] Serial number <![CDATA[Na2CO3 content / %]]> NaOH content / % Example 1 0.612 0.191 Example 2 0.520 0.162 Example 3 0.498 0.153 Comparative Example 1 0.985 0.263

[0136] As can be seen from Table 1, the coating treatment helps to reduce the residual alkali on the surface of the material, thereby improving the cycle performance.

[0137] First charge and discharge test:

[0138] In a drying room with a dew point below -40°C, the prepared positive electrode material, binder, and conductive carbon black are mixed in NMP (N-methylpyrrolidone) in a mass ratio of 90:5:5, homogenized, and the solid content is controlled at 45%. The mixture is coated on an aluminum foil current collector, vacuum-baked at 100-110°C for 5-8 hours, pressed into shape, and then punched into a sodium positive electrode sheet. A button half-cell is assembled in an argon-filled glove box. The counter electrode is a metal sodium sheet, the separator used is PE, and the electrolyte is 1 mol / L NaPF6 EC / DMC (Vol 1:1). The button cell is subjected to charge and discharge tests. The button cell test equipment is the commercial LAND battery test system of Wuhan Landian Electronics Co., Ltd. The first reversible capacity and efficiency of the sodium positive electrode material in the embodiment and comparative example are measured.

[0139] The battery was charged and discharged at 2.0-4.0V and a rate of 0.1C. Figure 6 、 7 , 8 and 9 are the first charge and discharge test curves of Example 1, Example 2, Example 3 and Comparative Example 1. For specific test results, see Table 2. After coating, the capacity of the layered oxide positive electrode materials obtained in Example 1, Example 2 and Example 3 decreased slightly.

[0140] Table 2 Charge and discharge performance of the embodiments and comparative examples

[0141]

[0142]

[0143] Cyclic performance test:

[0144] The battery was tested on a commercial LAND battery testing system of Wuhan Landian Electronics Co., Ltd. in a constant temperature environment of 25°C, and cycled 5 times at 0.1C, 0.2C, and 0.5C respectively, and 10 times at 1.0C.

[0145] Figure 10 The cycle performance diagrams of sodium ion batteries at different rates provided in Examples 1 to 3 and Comparative Example 1 are shown. Figure 10 It can be seen that although the initial capacity of the sodium ion batteries provided by Examples 1, 2, and 3 is slightly lower than that of Comparative Example 1, their cycling performance at a rate of 1.0C is significantly better than that of Comparative Example 1. In particular, the capacity of Example 3 at a rate of 1.0C is higher than that of Comparative Example 1. The surface coating of the structurally stable shell P2 phase improves the reversibility and structural stability of the material under cycling conditions and improves the rate performance.

[0146] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A layered oxide having a mixed phase structure, characterized in that: It includes an O3 phase material and a P2 phase material coated on the surface of the O3 phase material; The chemical composition of the O3 phase material is NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2; The chemical composition of the P2 phase material is Na 2 / 3 Mn 0.7 Mg 0.3 O2; The P2 phase material is 1wt% of the O3 phase material; The preparation method of the layered oxide comprises the following steps: dispersing an O3 phase material in ethanol to obtain a solution A; Dissolve polyvinyl pyrrolidone in ethanol to obtain solution B; Adding the B solution to the A solution to form a PVP coating layer on the surface of the O3 phase material to obtain a complex solution; According to the formula Na 2 / 3 Mn 0.7 Mg 0.3 Theoretical composition of Na, Mn and Mg in O2: Sodium acetate, manganese acetate and acetate containing Mg element are dissolved in ethanol to obtain solution C; Under heating conditions, the C solution is added to the complex solution, and after the ethanol is completely volatilized, the obtained solid is calcined to generate a P2 phase material on the surface of the O3 phase material to obtain a layered oxide with a mixed phase structure.

2. The method for preparing the layered oxide according to claim 1, characterized in that: The following steps are involved: Dispersing the O3 phase material in ethanol to obtain solution A; Dissolve polyvinyl pyrrolidone in ethanol to obtain solution B; Adding the B solution to the A solution to form a PVP coating layer on the surface of the O3 phase material to obtain a complex solution; According to the formula Na 2 / 3 Mn 0.7 Mg 0.3 Theoretical composition of Na, Mn and Mg in O2: Sodium acetate, manganese acetate and acetate containing Mg element are dissolved in ethanol to obtain solution C; Under heating conditions, the C solution is added to the complex solution, and after the ethanol is completely volatilized, the obtained solid is calcined to generate a P2 phase material on the surface of the O3 phase material to obtain a layered oxide with a mixed phase structure.

3. The preparation method according to claim 2, characterized in that The preparation method of the O3 phase material comprises the following steps: 1 / 3 Fe 1 / 3 Mn 1 / 3 The theoretical composition of Ni, Fe and Mn in O2 is obtained by mixing a water-soluble salt of divalent Ni, a water-soluble salt of divalent Fe and a water-soluble salt of divalent Mn with water to obtain a mixed metal salt solution; The mixed metal salt solution, alkali metal hydroxide solution and ammonia water are mixed, and a coprecipitation reaction is carried out at a pH value of 10 to 12, followed by aging to obtain a hydroxide precursor; mixing the hydroxide precursor and a sodium source to obtain a solid mixture; Alternatively, the hydroxide precursor and a sodium source are mixed to obtain a solid mixture; The solid mixture is sequentially subjected to a first sintering, a heating, and a second sintering to obtain an O3 phase material; the temperature of the first sintering is 400-500° C.; the temperature of the second sintering is 800-950° C.; The mass of Na in the sodium source is relative to the mass of NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 The theoretical mass of Na calculated by O2 is 2~5% in excess.

4. The preparation method according to claim 2, characterized in that The total concentration of sodium acetate, manganese acetate and acetate containing Mg element in the C solution is (0.001~1) g / mL.

5. The preparation method according to claim 2, characterized in that The temperature of the heating condition is 60~80℃.

6. The preparation method according to claim 2, characterized in that The calcination temperature is 800-850° C., and the heat preservation time is 10-15 hours.

7. The preparation method according to claim 2 or 5, characterized in that The calcination is carried out in air or oxygen atmosphere.

8. The preparation method according to claim 2, characterized in that The mass ratio of polyvinyl pyrrolidone in the B solution to the O3 phase material in the A solution is 0.005~0.05:

1.

9. Use of the layered oxide according to claim 1 or the layered oxide prepared by the preparation method according to any one of claims 2 to 8 as a positive electrode material in a sodium ion battery.

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